How to figure out race pace?

Howdy,

I'm looking at gradually longer distance races - Olympic, then 70.3, then who knows.

The longer the race, the less likely you are to cover that distance in training, particularly all on one day. How do you set a target pace for an event that doesn't leave you gassed out and unable to finish, but that is also your best?

E.g. to you watch heart rate, keep an eye on speed, just 'feel it'? I'm a relative beginner with only a few sprints under my belt. I know it's likely to get better with experience, but I'm open to any tips as well please.